This study has aimed to compare the effects of two types of form-focused instruction, i.e. de-contextualized focus-on-forms instruction versus meaning-centered contextualized focus-on-form instruction, on the development of grammatical knowledge of Iranian high-school students. Two groups of male high-school first graders participated in this study. One group was taught through de-contextualized deductive grammatical explanation, while the other group received enhanced input and contextualized grammar instruction embedded in meaning-centered activity based on dialogs. The results indicated that de-contextualized instruction as it is normally practiced in high-school contexts failed to promote successful use of the auxiliaries do, does, and did in a written production test. In contrast, adding the design features of meaning-based contextualization and enhanced input to instruction did result in better performance on a written production grammar test involving the use of the auxiliaries under study. In the delayed posttest, contextualized meaning-based instruction appeared to have a durable effect, but the effect was not significant enough to warrant any claim for the durable superiority of this form of instruction.